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Definition of tas classification
A classification scheme for volcanic rocks based on their bulk chemical composition.
TAS stands for Total Alkali versus Silica.
Volcanic rocks often cannot be classified by their mineral composition because the rock is finely crystallized or even made of volcanic glass, and get classified by their chemistry.
Within the IUGS classification of igneous rocks this is done by determining the oxide composition of the rock and plotting the total of the alkali oxides (Na2O + K2O) against the total silica (SiO2) content in weight percent. The corresponding diagram is called TAS diagram.
The official field names and rock definitions can be found in the figures below.
TAS diagram - rock names 
TAS diagram field names and coordinates
TAS stands for Total Alkali versus Silica.
Volcanic rocks often cannot be classified by their mineral composition because the rock is finely crystallized or even made of volcanic glass, and get classified by their chemistry.
Within the IUGS classification of igneous rocks this is done by determining the oxide composition of the rock and plotting the total of the alkali oxides (Na2O + K2O) against the total silica (SiO2) content in weight percent. The corresponding diagram is called TAS diagram.
The official field names and rock definitions can be found in the figures below.
TAS diagram - rock names
TAS diagram field names and coordinates
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Locality Updated: Peleochora, Crete Department (Kriti), GreeceFrom Debbie Woolf, 22nd May 2013 00:06:10